Piezoresistive behavior of a stretchable poly(dimethylsiloxane) sheet with an interlayer of carbon nanotube in it (CNT-interlayered PDMS) was investigated based on two variables: individual length of CNT which forms the interlayer and degree of pre-tension of the sample during the preparation stage. The stress–strain curves exhibited good stretchable properties of the CNT-interlayered PDMS sheets, similar to that of neat PDMS sheets and better than that of CNT/PDMS composites. The electrical resistance changed almost linearly with applied strain and the results of piezoresistive experiments evidenced that these two variables are factors that indeed affect piezoresistive behavior. The results are explained by the pre-tension-derived wrinkled structure of the CNT layer and the effect of ultrasonication on the CNT length. The sensitivity is described by a gauge factor in the range 0.19–1.16. The samples show good conductivity even at large elongations (∼55%) and excellent properties in cyclic piezoresistive experiments at small (∼11%) as well as at large strain.